Our work is focussed on the molecular mechanisms responsible for establishing and maintaining stable states of gene expression during vertebrate embryogeneis. Progress has been achieved in the following key areas: 1. We have probed the internal architecture of a positioned nucleosome with photoactivatable cross-linking reagents to determine the placement of histones along the DNA molecule. We propose a new model of the nucleosome in which the winged-helix domain of the linker histone is asymmetrically located inside the gyres of DNA that also wrap around the core histones. 2. We have determined that HMG 1, linker histone B4 and H1 share a structural role in chromatin, yet have distinct functional consequences for the transcription of chromatin templates. These results explain the transcriptional promiscuity of early embryonic chromatin. 3. We have discovered that triplet repeat expansions associated with human disease genes including Fragile X Mental Retardation Gene 1 and the Dentatorubral-pallidoluysian Atrophy Gene share the common property of assembling nucleosomes of extraordinary stability. This stability might make a major contribution to the expansion of these repeats. 4. We have found the molecular chaperone nucleoplasmin to have a key role in remodeling somatic nuclei during their acquisition of totipotency in Xenopus eggs. 5. We have determined that a role for nucleosome assembly exists in both the silencing and activation of the Xenopus TRbetaA gene by the thyroid hormone receptor.